1. Field
This invention is in the field of semiconductor wafer probing and, more specifically, relates to alignment of a probe tip to a device under test.
2. Related Art
Semiconductor wafers generally undergo probing using a probe card which makes contact to conductive pads on top of the IC. As technology advances, probing of wafers becomes more important in order to ensure proper designs and acceptable yield. However, as technology advances, the number of pads increases while the size of individual pads and the pitch between pads decrease. This makes it much harder to ensure that all contacts of the probe card make proper contact to the corresponding pads.
Device probe pads are the topmost layer of a chip. Prior art techniques for PTPA (Probe To Pad Alignment) require an unobstructed view of such pads from the backside of the wafer, and an infrared camera to see them through the silicon substrate. However, modern fabrication techniques for semiconductors place many layers of interconnecting metal (as many as 10 or more) between the topmost and bottommost layers in a device. These metal layers block all visibility of probe pads from below and thus render such existing techniques inadequate.
FIGS. 1-3 illustrate prior art arrangements for probe pins alignment. In FIG. 1, wafer 100 is held by a chuck 105, which is attached to an x-y stage 110. A camera 115 is also attached to the x-y stage 110. To perform the alignment, the stage is scanned so that the camera 115 can take pictures of the probe tips 120 of probe card 125. The image is used to form a mapping of the tips' locations. In FIG. 2, an infrared camera 215 is used to image both the pads 202 on the wafer 200 and the probe tips 220 of probe card 225. As shown in FIG. 2, in position A, camera 215 is focused on the plane where the pads are, so that it images the pads 202 through the silicon wafer (hence the use of infrared, to which silicon is transparent). At position B, camera 215 is focused on the plane where pins 220 are, so that it images the pins 220, also through the silicon wafer. When no or very few metallic lines are fabricated on the wafer, the IR camera can image both the pads and the probe tip through the silicon. This is shown in FIG. 3, wherein the pads are shown as squares and the probe tips are shown as round. However, when several layers of metal lines are fabricated on the wafer, the lines block the view of the camera and prevents imaging the pads and/or the probe tips, so that proper alignment cannot be verified using this technique.
Therefore, new apparatus and methods are needed in order to ensure proper alignment of probe tips to contact pads, even when several layers of metal lines are fabricated on the wafer.